Some Abhe Letters. 237 



A Note by J. Eheinberg, F.E.M.S. 



By Professor Cliesliire's courtesy I have had an opportunity of 

 perusing the interesting unpublished letters of Abbe to Stephenson 

 — letters most interesting on account of the light they throw upon 

 Abbe's views at the time, and on the many controversies which 

 the Abbe diffraction theory led to. 



To rightly appreciate things, we have to try and throw ourselves 

 back to the time these letters were penned. Abbe was thirty-six 

 years old ; he had been eight years with Zeiss, and had been 

 working on the bases of his famous theory for the last few years. 

 The diffraction theory, or the theory of secondary imaging as they 

 preferred to call it abroad, which he had evolved, largely as the 

 result of a series of careful and in many ways startling experi- 

 ments on Gratings, immediately led to the most important theo- 

 retical and practical results, and although that theory was frequently 

 challenged, in its foundations and in its chief and main aspects, 

 it has stood the test of time, and may, I think, rightly be termed 

 unassailable. 



That, however, is not quite the case with all the deductions 

 that were made from the theory. Abbe himself drew certain 

 incorrect inferences from his own theory, notably that the use ot 

 wide-angled cones of light would not affect the image by yielding 

 a truer rendering of structure than the use of narrow-angled cones 

 would give, and Mr. E. M. Nelson did the greatest service to 

 microscopy by combating this particular opinion, and showing that 

 that was not so. Also in the early days, when Abbe brought out 

 his theory, a number of his partisans, who imperfectly understood 

 the theory, came out with positively fantastic statements — I will 

 only name one, viz. : that since the images were rendered by the 

 diffracted beams, in order to obtain the truest image it would 

 really be best to stop out the central or dioptric beam altogether. 



That an idea like this never entered Abbe's mind is clearly 

 shown, for instance, in the passage of his letter read to-day, in 

 which he says : — 



" In some passages of my letter I distinguish the pencil 

 of direct rays and the diffraction pencils, but this distinction 

 does not mean any principal difference in the function or 

 action of these rays. From a general point of view, the 

 pencil of direct rays is one among the diffraction pencils. 

 It is different from the others only by its greater intensity 

 of light, but in its action in the formation of images of 

 structural objects it is quite on the same range with the 

 others." 



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